How fungi helped create life as we know it

Today our world is visually dominated by animals and plants, but this world would not have been possible without fungi, say University of Leeds scientists.

Researchers
have carried out experiments where plants and fungi are grown in atmospheres
resembling the ancient Earth and, by incorporating their results into computer
models, have shown that fungi were essential in the creation of an oxygen-rich
atmosphere.

Humans
and other mammals require high levels of oxygen to function, and it is
generally thought that the planet developed an oxygen-rich atmosphere around
500 to 400 million years ago, as carbon dioxide was gradually photosynthesised
by the first land plants.

The new
research shows that the amount of phosphorus transferred could have been very
large under the ancient atmospheric conditions, and, using an "Earth system" computer model, the team show that fungi had the power to dramatically alter
the ancient atmosphere.

Vital relationship

While
many modern plants can gather their nutrients direct from soils through their
roots, the earliest forms of plant life faced an entirely different climate,
did not have roots and were non-vascular, meaning they could not hold water or
move it around their system.

The "soil" they came into contact with was a mineral product lacking in organic
matter, which is why their relationships with fungi were so important.

Fungi
have the ability to extract minerals from the rocks they grow on through a
process known as biological weathering. The fungi express organic acids which
help to dissolve the rocks and mineral grains they grow across.

By
extracting these minerals and passing them on to plants to aid the plants
growth, the fungi in return received the carbon the plants produced as
they photosynthesised carbon dioxide from the atmosphere.

Gas exchange

Lab
experiments undertaken by the Leeds team have shown that different ancient fungi,
which still exist today, conducted these exchanges at different rates, which
influenced the varied speeds at which plants produced oxygen.

In turn
this affected the speed at which the atmosphere changed from being much more
rich in carbon dioxide to becoming similar to the air we breathe today.

Dr Field said:
We used a computer model to simulate what might have happened to the climate
throughout the Palaeozoic era if the different types of early plant-fungal
symbioses were included in the global phosphorus and carbon cycles.

We found the
effect was potentially dramatic, with the differences in plant-fungal
carbon-for-nutrient exchange greatly altering Earths climate through plant-powered
drawdown of CO2 for photosynthesis, substantially changing the
timing of the rise of oxygen in the atmosphere.

Dr Mills
said: Photosynthesis by land plants is ultimately responsible for about half
of the oxygen generation on Earth, and requires phosphorus, but we currently
have a poor understanding of how the global supply of this nutrient to plants
works.

The
results of including data on fungal interactions present a significant advance
in our understanding of the Earths early development. Our work clearly shows
the importance of fungi in the creation of an oxygenated atmosphere.

Dr
Batterman added: Our study shows tiny organisms such as fungi can have major
effects on the global environment. Our critical finding was that the nature of
the relationship between fungi and plants could have transformed the atmospheric
carbon dioxide, oxygen and ultimately global climate in very different ways,
depending on the type of fungi present.